feat(helpdesk): add Domains list page with BC OData contract type enrichment

Add a new "Domains" page to the helpdesk module that fetches domain data
from the FS_Contract_Domains OData endpoint and enriches each domain with
its contract type (PI_Header_Type) by joining with FS_Contract_Lines_Test
where Type = 'Domain'.

New files:
- domains/index().php, index(default).phtml, filter-schema.php — list page
- domains-data().php — data endpoint with PHP-side filtering/sorting
- helpdesk-domains-index.js — Grid.js via initStandardListPage()

Gateway additions:
- listDomains() — fetch all contract domains
- listDomainContractLines() — fetch domain-type contract lines for type lookup

Sidebar restructured into three collapsible groups (Lookup, Monitoring,
Administration) with per-group icons and color coding, matching the core
admin panel pattern.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

modules/helpdesk/i18n/default_en.json

@@ -345,5 +345,16 @@
   "Upcoming meetings": "Upcoming meetings",
   "Past meetings": "Past meetings",
   "No meetings found for this customer.": "No meetings found for this customer.",
-  "Could not load meetings.": "Could not load meetings."
+  "Could not load meetings.": "Could not load meetings.",
+  "Domains": "Domains",
+  "Search domains...": "Search domains...",
+  "Customer No.": "Customer No.",
+  "Customer Name": "Customer Name",
+  "Administration": "Administration",
+  "No domains found": "No domains found",
+  "Could not load domains.": "Could not load domains.",
+  "Lookup": "Lookup",
+  "Monitoring": "Monitoring",
+  "Contract type": "Contract type",
+  "Customer": "Customer"
 }